#include <stdio.h>
#include <math.h>
#include <string.h>

int X1,Y1,X2,Y2,W,D,X,Y;
double P;

int l,r,f,b,first_l,first_f,last_l,last_f,N;
char c[10010];

int main()
{
    int i;
    scanf("%d %d %d %d %d %d",&W,&D,&X1,&Y1,&X2,&Y2);
    scanf("%s",c);
    N=strlen(c);
    for (i=0; i<N; ++i)
    {
        if (c[i]=='L') { first_l=1;    break; }
        if (c[i]=='R') { first_l=0;    break; }
    }
    for (i=0; i<N; ++i)
    {
        if (c[i]=='L') last_l=1;
        if (c[i]=='R') last_l=0;
    }
    for (i=0; i<N; ++i)
    {
        if (c[i]=='F') { first_f=1;    break; }
        if (c[i]=='B') { first_f=0;    break; }
    }
    for (i=0; i<N; ++i)
    {
        if (c[i]=='F') last_f=1;
        if (c[i]=='B') last_f=0;
    }
    for (i=0; i<N; ++i)
    {
        if ((c[i]=='L') || (c[i]=='R')) l++;
        if ((c[i]=='F') || (c[i]=='B')) f++;
    }
    if (l>0)
    {
        X=(l-1)*W;
        if (first_l==1) X+=X1; else X+=W-X1;
        if (last_l==1) X+=X2; else X+=W-X2;
    }
    else X=X1-X2;
    if (f>0)
    {
        Y=(f-1)*D;
        if (first_f==1) Y+=Y1; else Y+=D-Y1;
        if (last_f==1) Y+=Y2; else Y+=D-Y2;
    }
    else Y=Y1-Y2;
    P=sqrt((double)X*X+Y*Y);
    printf("%.4lf",P);
    return 0;
}

I think problem is this:
P=sqrt((double)X*X+Y*Y);
In 12th test 'int' will oveflow during multiplication.
Just think of __int64.